The present invention relates to a method and apparatus for vibration-filling electrodes in a paste bath. The invention is preferably used for vibration-filling round electrodes having welded-on current drain tabs, but can also be used for electrodes having a prismatic or other shape.
Methods and appliances for vertically vibration-filling electrode substrates with an active compound paste are disclosed for example, in German Patent Specification 3,822,197, German Patent Specification 3,816,232, and German Patent Specification 3,822,209. However, the implementation of the methods described therein for round electrodes having a tab angled perpendicularly to the electrode plane is problematical, especially if the tab is welded on at the edge of a central hole, as in the case of the so-called pineapple-slice design for nickel/hydrogen cells.
The tab should be welded to the empty substrate before filling the latter but should not be obstructive during filling. In the arrangements disclosed in the above mentioned publications, normal rectangular electrodes having comparatively robust sheet metal tabs are welded onto a reinforced edge of the electrode substrate. (See for example, German Patent Specification 3,142,091 and German Patent Specification 3,632,251.) These tabs can be used to fix the electrodes in the filling appliance.
On the other hand, the electrodes for nickel/hydrogen cells carry long tabs of thin nickel sheet, which bend easily under the weight of the electrode. It is therefore usual to provide such round electrodes before filling with small auxiliary tab, onto which the long current drain tabs are welded after filling. This procedure entails some serious disadvantages, however. First, there is a double welding operation. (The auxiliary tab must be welded onto the electrode, and after filling, the current drain tab must then be welded to the auxiliary tab.) It is also necessary first to clean the tab; and the handling of the filled electrode is particularly difficult. In addition there are risks of dust being given off, and of mechanical damage to the electrode. In practice, therefore, it is customary to first wet the filled and dried electrode again before welding on the tab to the auxiliary tab in order to prevent dust of the carcinogenic nickel hydroxide being given off. When this is done, a second, additional electrode drying operation is necessary.
The object of the present invention is to provide a method and apparatus for vibration-filling fibrous or expanded structure substrates of electrodes having a welded-on current drain tab, wherein the substrates are filled rapidly and without great expense in a paste bath in a continuous working operation, and are capable of being easily cleaned of the excess of paste. Another object of the invention is to provide such a method and apparatus wherein a satisfactorily reproducible filling of the electrode substrates is achieved.
These and other objects are achieved by the method according to the invention, wherein the tab is bent so that it extends out of the paste bath approximately at right angles to the electrode surface, and the filling is carried out in the horizontal position of the electrode. For this purpose, the empty substrate is clamped in a support which has a vertical rotation shaft, and holding devices for the substrate which leave most of the substrate surface free. The electrode can be fixed at its periphery, for example, by claws; in the case of electrodes having a central hole or other formed openings, the holding devices can engage in the openings.
The tab is supported on the shaft of the holding tool. A tab made of nickel or of another ferromagnetic material is expediently held by a permanent magnet or electromagnet mounted on the shaft or forming a part of the shaft. To release the tab, the electromagnet can be switched off. Other holding means for the tab are clips, hooks, brackets or inflatable expanding bodies. Regardless of whether the holding means fix the tab securely or only loosely, they should act on the upper third of the tab length, so that a further step in the method according to the invention is possible, namely the cleaning of the filled electrode by spinning off the excess paste. For this purpose, the holding tool with the electrode substrate is withdrawn again from the paste bath after the filling operation and rotated by means of the vertical shaft.
Expediently, the method according the invention is performed by a computer-controlled automatic machine which brings the holding tool, and consequently the electrode, to a rotary speed sufficient to clean the electrode without loss of paste from the interior of the electrode. In this process, the tab support described above prevents bending of the tab under the influence of the centrifugal force during spinning.
Filled electrodes are, as a rule, heavier than catalyst electrodes which carry only a relatively thin catalyst layer. This applies particularly to fibrous and expanded structure electrodes, which can be of thicker construction than sintered foil electrodes, and are therefore capable of accepting particularly high capacities per unit area. Moreover, when the filled electrode is removed from the paste, its weight is at first still increased by the adhering excess paste which has to be spun off. Depending on the electrode size and the viscosity of the paste, therefore, an appreciable motor power is required for the spinning operation.
In order to manage with small, low power motors, such as stepping motors, the spinning-off is carried out with at least two rotary speeds. A lower rotary speed is selected such that at least 30% of the excess paste, or if possible still more, is removed. A higher rotary speed then serves to remove the more firmly adhering excess surface layer. Such preliminary cleaning by pre-spinning also has the advantage that if the rotary speed, paste pot diameter and paste viscosity are correctly matched, much of the excess paste flows back directly into the paste bath because the centrifugal force is not sufficient to spin the paste onto the pot wall. The exclusive return of a relatively viscous paste via the pot wall involves the risk of drying and crust formation on the pot wall. To reduce this risk, a cover whose opening just allows the insertion of the substrate may be placed on the paste post while the internal diameter of the pot is kept larger at the level of the spinning zone in order to make possible drainage of the excess paste during pre-spinning without wall contact.
The horizontal vibration-filling, i.e., the filling of an electrode substrate arranged with its surface horizontal and parallel to the paste pot base, is best accomplished with very small amounts of paste, and with the substrate immersed as shallowly as possible in the paste in order to minimize any contamination of the tab and holding tool. Owing to the small amount of paste, the entire paste pot can be mounted on a vibrator and vibrated. The paste pot should be constructed deep enough to provide room for spinning off the excess paste after impregnation of the substrate has been carried out. A constant paste level can be provided by a metering device which is so controlled by the automatic impregnating machine that the paste pot is again topped up with an appropriate amount of paste after every filling. The diameter of the paste pot in the actual paste bath section should exceed the electrode diameter as little as possible in order to minimize the amount of paste that is required. The diameter of the paste container in the region situated above the paste bath is normally greater than the diameter in the lower region in which the paste bath is present. The diameter may then decrease again towards the opening of the container. The shape of the container may therefore deviate more or less from the cylindrical shape and even assume spherical shape. For non-round electrodes, angular paste pots can also be considered.
The immersion and withdrawal of the electrode to be pasted should be sufficiently slow that deformations of the electrode surface are avoided; a higher speed is recommended outside the paste bath for the sake of saving time.
According to the invention, a substantial improvement of the filling of the electrode can be provided by installing above the electrode a rigid vibration reflecting body which is immersed in the paste bath during filling. In the simplest and preferred design, the reflecting body is in the form of an electrode-parallel plate, which reflects the vibrations originating from the pot base. This results in an intensified fluidization of the paste in the vibration field and consequently in more rapid and complete filling of the electrode substrate. The reflector may be a component of the electrode holding tool, or may be fixed to the latter. However, in principle it may also be a separate part of the apparatus, and be positioned by separate aids. Attachment of the reflector to the holding tool is preferred, because it achieves a simultaneous cleaning of electrode and reflector plate during spinning-off and avoids equipment expenditure for the purpose of reproducibly positioning the reflector.
The reflector should not only be rigid, but also light; and its surface should by hydrophobic. All these properties are offered by a plastic plate constructed of, for example, polystyrene. The shape of the plate should roughly correspond to that of the electrode. Thus, for example, in the case of round electrodes having a central hole as mentioned above, a similarly shaped reflector plate is beneficial. To feed through the above mentioned bent tab, the reflector has an opening, (or in the simplest case a slot), which extends to the edge of the plate.
It is obvious that the requirement for rigidity of the reflector need not be absolute, since even elastic bodies (for example, spring steel sheet) can reflect the shockwaves originating from the pot base, albeit with loss of energy. The method can therefore be implemented with reduced quality even with non-rigid reflectors. Thus, as used here, the description "rigid" means only that no substantial deformation of the reflector occurs during the vibration.
During vibration-filling, air escapes from the electrode substrate and rises in the paste bath. This air has to be removed as rapidly as possible, since it brings about an appreciable vibration damping. To deaerate the paste rapidly in the space between electrode substrate and reflector, the reflector may be provided with further openings in addition to the opening intended for feeding the tab through; for example it may be constructed as a perforated plate. This measure impairs, however, the reflection properties of the reflector under some circumstances. The design of the reflection body therefore must be matched in each case, therefore, to the particular system properties, which are determined by the vibration spectrum and the viscoelastic properties of the paste, and also by the geometry of the arrangement in the paste pot. The distance of the reflector from the electrode substrate is determined by the need to degas the paste rapidly and to clean the electrode and reflector by spinning-off. During spinning-off, the paste (which is no longer fluidized by vibration) must leave the gap between electrode and reflector satisfactorily and be capable of being spun onto the pot wall. In practice, a distance of 1 to 14 mm, and especially 4 to 12 mm, has proved advantageous.
It may sometimes be beneficial to provide the reflector with struts, ribs, pins or other surface structures which serve as electrode spacers and are able to make contact with the electrode at its surface. (Radially extending ribs are preferred for this purpose.) Where such reflectors make contact with the electrode by means of projections, care must be taken that as large a part of the electrode surface as possible remains untouched. Reflectors formed in this way can be concomitantly used to support the substrate or the electrode by forming a thrust block for clamping the electrode. Conversely, they may also utilize the electrode as thrust block for supporting, for example, a hinged reflector design.
To degas more rapidly and distribute the air bubbles better, it has been found advantageous to subject the electrode to an oscillation around the vertical axis during the impregnation. For this purpose, the rotary drive for the spinning-off is utilized. The drive should therefore make oscillatory movements possible; a stepping motor, for example, is suitable. The oscillation angle does not need to exceed 30.degree. and the frequency should be above 5 Hz.
Although the filling of substrates is possible in a wide range of frequencies and amplitudes (40 to 125 Hz and 0.1 to 1.55 mm) for the paste container, for horizontal impregnation in accordance with the method according to the invention it is advisable not to exceed an amplitude of 0.2 mm, and to set the peak frequency at least 100 Hz to avoid splashes. Peak frequency is understood here to mean the highest frequency occurring during the vibration cycle.
The invention can also be utilized to fill normal rectangular electrodes having an outer edge tab, if the tab length is not prohibitively large. (This applies, in general, to electrodes of sealed cells, such as Ni/Cd cells having a prismatic housing.) Expensive cleaning of the tab is thus not necessary, since even if it is immersed in the paste, the tab is cleaned by the centrifugal force while the electrode is spun. Since the rectangular electrodes may be appreciably larger than the conventional round electrodes, it is advisable under some circumstances not to attach the reflection plate to the holding tool, but to locate it separately and to arrange it above the substrate after it has been inserted in the paste. For example, the reflection plate may be constructed as a hinged part which is swivelled upwards during insertion of the substrate into the paste bath and during its removal from the paste bath. This arrangement also has the advantage that the paste spun off is intercepted by the swung-up plate and is introduced into the paste bath during the next filling cycle. In the design as a hinged or folding part, the reflection plate may also be divided.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.